1. Field of the Invention
This invention relates to an ink ribbon cassette adapted to be removably mounted with respect to a recording apparatus such as a printer and to contain an ink ribbon therein.
2. Related Background Art
Generally, an ink ribbon cassette contains therein a supply side ribbon spool and a take-up side ribbon spool for winding an ink ribbon thereon. There are available ink ribbon cassettes in which the take-up side ribbon spool is movable.
A ribbon cassette in which a take-up spool is movable is shown in FIGS. 15A and 15B of the accompanying drawings. In these figures, a ribbon 101 drawn out from a supply ribbon spool 100 is exposed outwardly and again introduced into the cassette, and then wound onto a take-up ribbon spool 102. The take-up spool 102 is rotatably supported on one end of a support arm 103. The other end of the support arm 103 is biased in the direction of arrow L with a force G by a coil spring 104.
The take-up spool 102 biased in the direction of arrow L is moved toward an opening 105 formed in a wall surface of the ribbon cassette. The take-up spool 102 thus moved strikes against a side wall 106 of the opening 105 and is stopped thereby.
On the other hand, when feeding of the ink ribbon is to be effected, a feed tooth 107 on the recording apparatus side is moved in the direction of arrow M to push back the take-up spool 102 in the direction opposite to the direction L against the force of a spring 104 and separate the take-up spool 102 from the side wall 106. In this state, the feed tooth 107 is rotated by a drive source, not shown, to take up the take-up spool 102.
On the other hand, when take-up is not effected, the take-up spool 102 is in contact with the side wall 106, as previously described. FIG. 16 of the accompanying drawings shows such state, and the take-up spool 102 is in contact with the side wall 105 by a surface 108.
If the take-up spool is left in the state of FIG. 16 for a long time, a stepped portion 109 is longitudinally formed on the surface of the take-up spool 102 as shown in FIG. 17 of the accompanying drawings. This is because the ink ribbon on the spool 102 is dented. The presence of such a stepped portion 109 would cause tooth skip or the like when an attempt is made to rotate the take-up spool by the feed tooth 107, thus resulting in inaccurate take-up of the ribbon.
Further, when the force applied between the take-up spool 102 and the side wall 106 is considered, there are numerous disadvantages.
These will now be described with reference to FIGS. 18 and 19 of the accompanying drawings.
FIG. 18 shows the start of the take-up of the take-up spool 102, and FIG. 19 shows the course of the take-up of the take-up spool 102. The significances of symbols shown in the figures are as follows:
r.sub.n : radius of the ink ribbon PA0 G.sub.n : spring force PA0 G'.sub.n : spring force in the direction of the normal which acts on side wall 106 PA0 P.sub.n : reaction force received by take-up spool 102 PA0 .beta..sub.n : the angle between the directions in which Gn and G'n act PA0 n=1: initial stage PA0 n=n: take-up is going on PA0 n=f: end of take-up PA0 l: distance from side wall 106 to a straight line lying on the same axis as the direction in which the spring force acts
Now, from FIG. 19, ##EQU1## .beta.n is eliminated from (1) and (2), and thus, ##EQU2##
G'.sub.n is equal in absolute value to P.sub.n which is the reaction force the ink ribbon receives from the side wall 106, and this reaction force serves to regulate the ink ribbon wound on the take-up spool 102 so that the ink ribbon does not become slack. Further, generally, EQU P.sub.1 .ltoreq.P.sub.n ( 7)
and therefore, P.sub.1 is the minimum necessary reaction force for preventing the slack of the take-up spool.
From the foregoing, G.sub.n and P.sub.n may be graphically shown as in FIG. 14 of the accompanying drawings. As is apparent from FIG. 14, when the turn radius of the take-up spool 102 is minimum, the component G'.sub.1 of the spring force G.sub.1 in the direction toward the side wall 106 is diminished to about one half of G.sub.1. This is because the side wall 106 contacts the take-up spool 102 not at the central point 110 of the take-up spool 102 but at a point off the central point 110. If G'.sub.1 thus becomes small as compared with G.sub.1, G.sub.1 itself must be made great to sufficiently satisfy the minimum reaction force P.sub.1 necessary for preventing the slack of the take-up spool 102. For this reason, in the prior art, use had to be made of a spring 104 of very great spring constant.
The use of such a spring of great spring constant to bias the take-up spool 102 unavoidably leads to the necessity of increasing the strength of the entire ribbon cassette, which in turn leads to an increased cost. There is also a danger that movement of the take-up spool 102 cannot be accomplished smoothly.
Also, in this example of the prior art, the spring force G.sub.n is applied in the same direction as the direction of movement of the take-up spool 102 and therefore, G.sub.n increases in proportion to the increase in the turn radius of the take-up spool 102. If G.sub.n thus increases in proportion to the amount of turns, when the feed tooth 107 eats into the take-up spool 102 during the feeding of the ribbon, the eat-in force will become greatly irregular depending on the turn radius of the take-up spool 102. If this eat-in force becomes irregular, the amount of eat-in of the feed tooth will vary, thus causing a disadvantage that the amount of feed of the ribbon varies depending on the turn radius of the take-up spool.
Further, as the turn radius increases, the angle .beta..sub.n between G.sub.n and G'.sub.n decreases. Thus G'.sub.n /G.sub.n gradually increases as the turn radius increases.
For this reason, as can be seen from FIG. 14, the reaction force P.sub.n the take-up spool 102 receives from the side wall 106 sharply increases due to the increase in the turn radius. Such a sharp increase in P.sub.n is not preferable because it remarkably expedites the formation of the stepped portion 109 shown in FIG. 17.
FIG. 20 of the accompanying drawings shows another example of the prior art. A supply side ribbon spool 201 and a take-up side ribbon spool 202 are contained in a cassette 200. Reference numerals 203 and 204 designate manually rotatable knobs. Feeding of the ribbon can be accomplished by holding these knobs.
FIG. 21 of the accompanying drawings shows a cross-section taken along line XX--XX of FIG. 20. The take-up side ribbon spool 202 comprises a take-up core 205 and an ink ribbon wound thereon, and the manually rotatable knob 204 is formed above and integrally with the core 205. Such integral formation of the knob 204 with the core 205 is preferable in that the number of parts is reduced.
However, if the knob 204 is formed integrally with the core 205, the diameter 207 of the knob 204 must be made smaller than the diameter 208 of a hole 206 for the purpose of assembly. Such a smaller diameter of the knob 204 leads to great difficulties in operation.
FIG. 22 of the accompanying drawings shows still another example of the prior art.
Recently, the amount of ink ribbon used has been increased, and this leads to the tendency of the ribbon cassette 300 toward bulkiness.
Such bulkiness of the cassette in turn leads to great difficulties in holding the cassette.
The bulkiness of the cassette also leads to the corresponding bulkiness of a ribbon feeding device 301, and this is not preferable.